ci: include the built-in `git add -i` in the `linux-gcc` job

This job runs the test suite twice, once in regular mode, and once with
a whole slew of `GIT_TEST_*` variables set.

Now that the built-in version of `git add --interactive` is
feature-complete, let's also throw `GIT_TEST_ADD_I_USE_BUILTIN` into
that fray.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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